It may be important in an electrical or electronic device to conduct heat away from modules, enclosures, circuit boards, integrated circuit chips, and other components and towards a metal plate or other heat sink element, for the effective dissipation of heat generated during operation.
Greases and pastes filled with thermally conductive fillers have been used for such purpose. However, they tend to migrate into adjacent spaces over time, particularly at elevated temperatures, contaminating other areas of the device and causing a loss of the desired thermal conductivity. They are also difficult to handle, particularly when re-entering the device for repair or replacement, because they are difficult to clean from surfaces on which they have been placed.
Alternatives to greases and pastes are thermally conductive gels or pressure sensitive adhesives, such as disclosed in Dittmer et al., U.S. Pat. No. 4,852,646 (1989); Mercer et al, WO 96/23007 (1996); and Chiotis et al., WO 96/05602 (1996). Because they are high elongation, low modulus (soft) materials, gels are highly conformable, allowing them to establish excellent thermal contact with irregular surfaces. Gels offer the advantage of facile re-enterability: they generally have a cohesive energy greater than their bonding energy to the surface onto which they have been applied, allowing them to debond cleanly. Many gels are made from crosslinked polymers systems, so that they will not migrate, unlike greases and pastes. (Gels made from a thermoplastic base polymer are also known; such gels also will not migrate provided the service temperature is kept below the melting temperature of the base polymer.)
A thermally conductive gel-based composition is made by filling the gel with a thermally conductive filler such as particulate silicon nitride, aluminum nitride, boron nitride, or alumina (aluminum oxide). The nitrides, especially aluminum nitride, are desirable because their high specific thermal conductivities enable their use in relatively smaller amounts while still achieving a desired high thermal conductivity in the resulting composition. However, aluminum and boron nitrides are more expensive than alumina by about two orders of magnitude, limiting their commercial utility. Also, we have discovered that aluminum nitride is hydrolytically unstable and will react with ambient moisture. The hydrolysis of aluminum nitride can cause a degradation in composite properties. In addition, the ammonia generated in the hydrolysis of aluminum nitride can accelerate corrosion and cause degradation of materials it comes in contact with and can interfere with the cross-linking of gel systems such as organopolysiloxane crosslinked via hydrosilylation chemistry.
Alumina's lower specific thermal conductivity (compared to the nitrides) means that a larger amount must be used in order to attain the same final thermal conductivity in a filled gel. Nevertheless, the cost and storage stability advantages of alumina make it attractive as a thermally conductive filler. Where the thermal conductivity required in the final gel is relatively low, the need to employ larger amounts of alumina is not a serious limitation. However, where the final gel needs to be highly thermally conductive (meaning, for the purposes of this specification, a thermal conductivity of at least 1.3 watt/m-.degree. C.), the large amount of alumina required (generally in excess of 60 weight percent) adversely affects the elongation and softness of the final product, compromising its conformability.